1. Field of the Invention
The present invention relates to bonding methods and, more particularly, to an anodic bonding method using a sol-gel solution for the bonding of various substrates, such as metals, glass, ceramics, and semiconductor materials.
2. Description of Related Art
Screen printed frit bonding techniques have been used for bonding silicon-based devices that have been micromachined. That technique involves a thick film paste bonding medium which is distributed through a photo-processed stencil or mask by a screen printer onto a wafer surface. The mask is needed to accomplish a uniform layer of the paste and the process does not typically allow for a layer over the entire substrate surface. With less than the entire surface having a bonding layer, there is less than maximum potential device yield per wafer.
Another past method of bonding substrates utilizes a soldering process with a solder having a lower melting point than the substrates. For example, Dalton, "Solder Glass Sealing," Journal of the American Ceramic Society, Vol. 39, No. 3, pp. 109-112 (1956) describes the matching of lead, silicon, and boron oxide solders for bonding glass substrates together. Generally, when the solder is heated and melted, it adheres to the substrates. Thereafter, the solder is cooled and solidified to form a mechanical bond between the substrates. However, the strength of the bond is dependent upon how well the solder adheres to the substrates and the strength of the solder itself. The degree of adherence is dependent upon the degree of contraction of the substrate relative to the solder during the solidification of the solder. A greater relative difference in contraction will cause a greater weakening of the bond. Accordingly, the strength of the bond is dictated by how well one can match the contractions of the materials being employed.
Anodic or field assisted bonding has been employed to bond various substrates. That method has been desirable because of, among other things, the relatively low temperatures employed, which tend to minimize the potential of substrate damage. As an example, anodic bonding of silicon to Pyrex.RTM. glass is described by Albaugh et al., "Mechanisms of Anodic Boding of Silicon to Pyrex.RTM. Glass," IEEE Solid State Sensor and Actuator Workshop, pp. 109-110 (1988). Traditionally, an ionically conducting dielectric layer is established between an insulating substrate such as glass or ceramic and a conducting substrate such as metal. A constant voltage is then applied across the insulating substrate. Thereby, a contact area between the substrates increases due to electrostatic attraction between the substrates. Next, permanent bonding forces develop by virtue of an ionic depletion layer developed in the dielectric layer at the interface to the insulating substrate. An oxygen depletion layer develops in the conducting substrate at the interface to the dielectric layer. The permanent bonding is chemical in nature.
Despite the advantages of anodic bonding, a disadvantage has been the manner in which the dielectric layer has been provided. Prior methods have used sputtered or chemical vapor deposition layers. But those methods have been time consuming because of the need for masks and batch processing. An additional processing constraint is present due to the typical need for a vacuum environment in those methods.
Other past anodic bonding methods have employed plates or shims for the dielectric layer. The plates and shims are commercially produced and have commonly been about 5 to 10 mils thick. Because of mechanical constraints on how thin a plate or shim can be machined, the applied voltage needed has also been constrained at a minimum amount. That is due to the fact that, for the same amount of bonding, as the thickness of the plate/shim increases, so does the need for higher applied voltages, and vice versa. And as the feature size in semiconductor devices decreases, the potential for device damage increases for the same applied voltage. With a Pyrex.RTM. glass shim, for example, which is about 10 mils thick, 20 to 30 V at 200 degrees C may be required to create an anodic bond to a conducting metal or semiconductor substrate. However, many semiconductors, such as a 0.5 .mu.m complementary metal on silicon (CMOS), usually have a limit of about 5 to 10 V before damage occurs, thus making the use of a typical Pyrex.RTM. glass shim impractical for prior anodic bonding techniques.
As can be seen, there is a need for an improved method of bonding substrates, such as by anodic bonding. An additional need is for a flexible bonding method such that various substrates can be employed. Another need is for an anodic bonding method that minimizes the potential for damage to the substrates being bonded. There is also a need for minimizing the thickness of a dielectric layer in an anodic bonding method such that the required applied voltage can be minimized.